This simply changes the constant '4' to MSG_PROTO_BYTES, which
includes the postamble. This helps reduce magic constant values
used in processing a packet.
BUG=chrome-os-partner:8975
TEST=tested on daisy (with kernel mkbp driver change)
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Change-Id: Id4634076ad63f45783354179dfebea4fd450fc1e